This invention relates to a mass flow rate/density sensor working on the Coriolis principlexe2x80x94hereinafter referred to as a Coriolis mass flow rate/density sensorxe2x80x94for measuring the mass flow rate and/or the density of a fluid which flows at least temporarily through a pipe; the Coriolis mass flow rate/density sensor is inserted into the pipe and mounted therein in a pressure-tight manner, for instance via flanges.
WO-A 99/51946 corresponding to U.S. patent application Ser. No. 09/283,401 filed Apr. 1, 1999, particularly in connection with FIGS. 9 and 10, discloses a Coriolis mass flow rate sensor, which, of course, is also a density sensor, and which is designed to be installed in a pipe through which a fluid flows at least temporarily, with a single measuring tube bent in a plane with respect to an axis of symmetry and comprising:
a straight inlet section with an inlet axis lying in the plane;
a straight outlet section with an outlet axis lying in the plane and aligned with the inlet axis;
an inlet bend;
an outlet bend;
a vertex bend,
the inlet section being seamlessly connected with the inlet bend, and the outlet section being seamlessly connected with the outlet bend, and
the inlet bend merging seamlessly into the vertex bend, and the vertex bend merging seamlessly into the outlet bend,
onto which inlet section a first and a second clamping body are clamped opposite each other to define a first limit of a measuring length forming a tube section of the measuring tube, and
onto which outlet section a third and a fourth clamping body are clamped opposite each other to define a second limit of the measuring length,
each of the clamping bodies having an internal surface, which rests against the measuring tube, and an external surface, which is remote from the internal surface and the measuring tube,
said first and second clamping bodies having an inlet-side first sensor support fixed thereto, a longitudinal axis of which is parallel to the inlet axis, and
said third and fourth clamping bodies having an outlet-side second sensor support fixed thereto, a longitudinal axis of which is parallel to the outlet axis,
said vertex bend having a seismic exciter fixed thereto
which excites the tube section in a third mode of vibration at an associated natural frequency f3 which, if the tube section is filled with the fluid, lies between approximately 500 Hz and 1000 Hz,
a first velocity or displacement sensor being fixed to the first sensor support and the inlet bend and a second velocity or displacement sensor being fixed to the second sensor support and the outlet bend at positions where, if the tube section is excited in the third mode of vibration, a deflection of the tube section caused by a disturbance originating from the pipe has a first and a second zero, respectively.
The design principle of this prior-art Coriolis mass flow rate sensor only permits vertex bends that have a large radius of curvature, or in other words, where the distance between the vertex and the inlet/outlet axis is only on the order of about 10 cm. For greater distances, particularly for distances greater by an order of magnitude than the order of 10 cm, the design principle disclosed in WO-A 99/51946 is unsuitable.
It is therefore an object of the invention to provide a Coriolis mass flow rate/density sensor which is based on the design principle described in the above prior art and in which the distance between the vertex of the vertex bend and the inlet/outlet axis can be practically arbitrarily great. At the same time, high measurement accuracy, e.g., an accuracy of the order of xc2x10.5%, is to be attainable.
To attain these objects, a first variant of the invention consists in a Coriolis mass flow rate/density sensor to be installed in a pipe through which a fluid flows at least temporarily, said Coriolis mass flow rate/density sensor comprising a single, V-shaped measuring tube bent in a plane with respect to an axis of symmetry and comprising:
a straight inlet section with an inlet axis lying in the plane;
a straight outlet section with an outlet axis lying in the plane and aligned with the inlet axis;
an inlet bend;
an outlet bend;
a vertex bend,
the inlet section being seamlessly connected with the inlet bend, and the outlet section being seamlessly connected with the outlet bend;
a straight first tube section, which seamlessly connects the inlet bend with the vertex bend; and
a straight second tube section, which seamlessly connects the outlet bend with the vertex bend,
onto which first tube section a first and a second clamping body are clamped opposite each other near the inlet bend to define a first limit of a measuring length forming a tube section of the measuring tube,
onto which second tube section a third and a fourth clamping body are clamped opposite each other near the outlet bend to define a second limit of the measuring length,
each of the clamping bodies having an internal surface, which rests against the measuring tube, and an external surface, which is remote from the internal surface and the measuring tube,
which external surfaces of the first and third clamping bodies have a first flat body attached thereto, and
which external surfaces of the second and fourth clamping bodies have a second flat body attached thereto,
which two flat bodies are screwed together and to the clamping bodies, with a first spacing element interposed at a first long side and a second spacing element interposed at a second long side,
which two flat bodies have an opposed-action body fixed thereto which extends along the axis of symmetry up to the vertex bend, where it supports a first portion of an exciter assembly, which has a principal axis and a second portion of which is fixed to the vertex bend,
which exciter assembly excites the tube section in a third mode of vibration at an associated natural frequency f3,
to which first flat body are fixed an inlet-side first sensor support, a longitudinal axis of which is parallel to the first tube section, and an outlet-side second sensor support, a longitudinal axis of which is parallel to the second tube section, and
to which second flat body are fixed an inlet-side first mount, a longitudinal axis of which is parallel to the first sensor support, and an outlet-side second mount, a longitudinal axis of which is parallel to the second sensor support,
a first velocity or displacement sensor being fixed to the first tube section and the first sensor support and a second velocity or displacement sensor being fixed to the second tube section and the second sensor support at locations where, if the tube section is excited in the third mode of vibration, a deflection of the tube section caused by a disturbance originating from the pipe has a first and a second zero, respectively, and
said inlet section and said outlet section being held by a supporting frame
to which a housing is fixed which is attached to the mounts by means of a first spacer, lying opposite the first velocity or displacement sensor, and a second spacer, lying opposite the second velocity or displacement sensor, respectively.
To attain the above objects, a second variant of the invention consists in a Coriolis mass flow rate/density sensor to be installed in a pipe through which a fluid flows at least temporarily, said Coriolis mass flow rate/density sensor comprising a single, V-shaped measuring tube bent in a plane with respect to an axis of symmetry and comprising:
a straight inlet section with an inlet axis lying in the plane;
a straight outlet section with an outlet axis lying in the plane and aligned with the inlet axis;
an inlet bend;
an outlet bend;
a vertex bend,
the inlet section being seamlessly connected with the inlet bend, and the outlet section being seamlessly connected with the outlet bend;
a straight first tube section, which seamlessly connects the inlet bend with the vertex bend; and
a straight second tube section, which seamlessly connects the outlet bend with the vertex bend,
onto which first tube section a first and a second clamping body are clamped opposite each other near the inlet bend to define a first limit of a measuring length forming a tube section of the measuring tube,
onto which second tube section a third and a fourth clamping body are clamped opposite each other near the outlet bend to define a second limit of the measuring length,
each of the clamping bodies having an internal surface, which rests against the measuring tube, and an external surface, which is remote from the internal surface and the measuring tube,
which external surfaces of the first and third clamping bodies have a first flat body attached thereto, and
which external surfaces of the second and fourth clamping bodies have a second flat body attached thereto,
which two flat bodies are screwed together and to the clamping bodies with the interposition of a first spacing element at a first long side and of a second spacing element at a second long side,
to which two flat bodies an opposed-action body is fixed which extends along the axis of symmetry toward, and ends before, the vertex bend,
to which first flat body are fixed an inlet-side first sensor support, a longitudinal axis of which is parallel to the first tube section, and an outlet-side second sensor support, a longitudinal axis of which is parallel to the second tube section, and
to which second flat body are fixed an inlet-side first mount, a longitudinal axis of which is parallel to the first sensor support, and an outlet-side second mount, a longitudinal axis of which is parallel to the second sensor support,
which vertex bend has a seismic exciter fixed thereto
which excites the tube section in a third mode of vibration at an associated natural frequency f3,
a first velocity or displacement sensor being fixed to the first tube section and the first sensor support, and a second velocity or displacement sensor being fixed to the second tube section and the second sensor support at locations where, if the tube section is excited in the third mode of vibration, a deflection of the tube section caused by a disturbance originating from the pipe has a first and a second zero, respectively, and
said inlet section and said outlet section being held by a supporting frame
to which a housing is fixed which is attached to the mounts by means of a first spacer, lying opposite the first velocity or displacement sensor, and a second spacer, lying opposite the second velocity or displacement sensor, respectively.
According to a first development of the two variants of the invention, a first added material is fixed to the first tube section near the vertex bend approximately where a node of the third mode of vibration occurs, and a second added material is fixed to the second tube section symmetrically with respect to the axis of symmetry.
In a first preferred embodiment of the invention and/or of its first development, the exciter assembly is fixed to the vertex bend and the opposed-action body in such a way that the principal axis of the exciter assembly extends in the direction of a vertex bend diameter vertical to the axis of symmetry.
In a second preferred embodiment of the invention and/or of its first development, the exciter assembly is fixed to the vertex bend and the opposed-action body in such a way that a principal axis of the exciter assembly extends parallel to a vertex bend diameter vertical to the axis of symmetry and lies between the vertex bend and the housing.
According to a second development of the two variants of the invention, a compensation body, a longitudinal axis of which is perpendicular to the axis of symmetry and which serves to dynamically balance a Coriolis mode belonging to the third mode of vibration, is fixed to the opposed-action body near the flat bodies.
According to a third development of the two variants of the invention, the two flat bodies and the first spacing element are provided with a first recess along the axis of symmetry, and the two flat bodies, the second spacing element, and the opposed-action body are provided with a second recess along the access of symmetry, leaving respective torsion portions.
In a preferred embodiment of the third development of the invention, the torsion portions are designed as a common swivel joint.
In a preferred embodiment of the two variants of the invention as well as of their developments, the first, second, third, and fourth clamping bodies have the same mass.
In another preferred embodiment of the two variants of the invention as well as of their developments, the housing is composed of flat metal sheets and comprises:
a front sheet with a first middle plane;
a rear sheet with a second middle plane;
a vertex sheet with a third middle plane;
a first side sheet with a fourth middle plane; and
a second side sheet with a fifth middle plane, with
the first middle plane being parallel to the first tube section,
the second middle plane being parallel to the second tube section,
the third middle plane being perpendicular to the plane,
the fourth and fifth middle planes being parallel to the flat bodies, and
the rear sheet being fastened via the spacers to the mounts.
One advantage of the invention is that it allows the construction of Coriolis mass flow rate/density sensors whose overall length, i.e., the length along the inlet/outlet axis, is considerably less than the overall length of the prior-art arrangement disclosed in WO-A 99/51946. This is due to, among other things, the V-shape of the measuring tube. A compact sensor with the desired measurement accuracy is thus obtained.
Another advantage accrues because an effect that is possible with the prior-art arrangement does not occur. This effect consists in the fact that, because of the clamping bodies being mechanically interconnected only via the measuring tube, but otherwise being independent, the measuring tube can be excited by vibrations originating from the pipe into vibrations that have a lower frequency than the frequency of the third mode. These low-frequency vibrations, like the vibrations of the third mode, are converted by the sensors into electric signals and are contained in the latter as interfering signals. In the arrangement according to the invention, the clamping bodies on the inlet side are substantially rigidly connected with those on the outlet side via, among other things, the flat bodies, so that the excitation of the low-frequency vibrations is practically impossible.